The invention relates to a cryopump with a housing that incorporates an intake opening, a two-stage cooling head in the housing, at least one cooling surface that is connected to the two-stage cooling head, a screen, and a shield that is connected to the first stage to act as radiation protection for the cooling surface of the second stage.
Cryopumps for vacuum technology generally have two levels with a cooling surface. With few exceptions all gases are condensed on this cooling surface. As a rule an outer cooling surface that acts as a shield against heat radiation is kept at about 80.degree. K., and an interior cooling surface is kept at a temperature level of 20.degree. K. With the exception of an opening for the input of gases, the shield surrounds the interior cooling surface. This opening is enclosed by a diaphragm consisting of segments. The diaphragm reduces the radiation of heat and thus reduces the thermal load on the interior cooling surface. The temperature level of the interior cooling surface would be elevated about 20.degree. K. with a direct exposure to heat. That would interfere with the absorption of gases with low molecular weight in such a way that the desired pump action could not be achieved for those gases.
Conventional diaphragms are made of concentrically placed sheet-metal rings, also known as chevrons. These are sheet-metal baffles of parallel, V-shaped sheet-metal strips that do not optically seal the interior cooling surface. Such diaphragms have the disadvantage that they impede the transport of the gas the disadvantage that they impede the transport of the gas molecules that are to be condensed or absorbed. On the other hand, a resistance-free transport of the gas molecules--in this case this is considered a high conductance--would result in maximum suction capacity of the cryopump.
In prior art cryopumps, the diaphragms were shaped to obtain an optimal compromise between admitting minimal heat radiation on the one hand and on the other hand obtaining maximum conduction, so that the cryopump would retain an equivalent suction capacity. However, a disadvantage of prior art cryopumps is that their suction capacity is considerably below the theoretically possible value. Cryopumps of the prior art also have the disadvantage that they have an extended structural, or construction, length or a large structural depth.
Many vacuum processes; for example, sputtering, use process gases. In such applications, an adjustable, stepless reducing valve is generally placed between the cryopump and the vacuum chamber so that the cryopump is not overloaded by high gas flow. The reducing valve is controlled by corresponding vacuum pressure sensors to seal the access of gas to the cryopump during specific process steps. However, that has the disadvantage that vapors harmful to the vacuum process, such as water vapor, cannot continually be pumped off in adequate quantity from the cryopump out of the vacuum process chamber. Such a valve therefor presents a tremendous disadvantage.